Without limiting the scope of the invention, its background will be described in relation to a system and method for purifying air via low-energy, in-situ regenerated silica-titania composites, as an example.
“Sick building syndrome,” used to describe acute negative health effects linked to time spent in a building, has been related to poor indoor air quality. Similarly, poor aircraft cabin air quality has been identified as a cause for negative health effects on pilots and flight crews, leading to numerous studies on “aerotoxic syndrome.” Symptoms of aerotoxic syndrome include headache, eye and nose irritation, cough, shortness of breath, chest tightness, increased heart rate, light-headedness, dizziness, blurred or tunnel vision, disorientation, confusion, memory impairment, shaking and tremors, loss of balance, vertigo, nausea, vomiting, seizures, and loss of consciousness. These symptoms can pose a serious threat if experienced by pilots and/or crew during flight.
Volatile organic compounds (“VOCs”) are a major source of indoor air pollution in aircraft and buildings. Sources of indoor VOCs include cleaners, building materials, furniture, human bioeffluents, cosmetics, food, and beverages, among others. In the case of aircraft cabins, ethanol is usually the VOC present at the highest concentrations.
In addition to the VOCs present in aircraft cabins due to off gassing from surfaces and human activities, the occurrence of “fume events” results in the release of more toxic gases, such as tricresyl phosphate, into aircraft cabins. Because engines have a source of compressed air for fuel combustion, this is a convenient source for providing compressed air to the cabin. Thus, air is bled from the engines upstream of the combustion chamber to supply the cabin air conditioning system. This bleed air, which is not filtered, may become contaminated with hydraulic oils prior to reaching the cabin. This leakage of hydraulic oil gases into the cabin is referred to as a fume event, and has been linked to the more severe symptoms of aerotoxic syndrome.
Technologies that can safely, economically, and effectively degrade VOCs from indoor air are necessary to protect human health. The most commonly used technology at present is activated carbon filters, which, although effective initially, quickly become saturated and must be disposed of and replaced. Additionally, carbon filters simply adsorb VOCs, rather than destroy them.
While the use of UV alone (i.e., photolysis) has been considered for this type of application, it may result in production of potentially hazardous oxidation intermediates if a photocatalyst is not present. Thin films of photocatalysts (e.g., titanium dioxide, also known as TiO2 or titania) on metal panels or other surfaces are a plausible option, but have a much lower surface area than the silica-titania composites described herein, and thus result in less effective oxidation. A combination of activated carbon, titania, and UV may also be used, but will also be less effective.
Ozonation has been considered, but excess ozone may be hazardous to human health. Other options include catalytic oxidation, which requires temperatures above 150° C. and may be poisoned by a fume event, and plasma oxidation, which results in generation of ozone and has critical residence time requirements to avoid production of oxidation intermediates.